In general, a seat belt system provided for a seat of a vehicle and the like is constructed so that webbing is wound into a retractor having an emergency locking device. In the event of a crash or the like, the emergency locking device operates to stop the rotation of a take-up drum around which the webbing is wound. This prevents the pay out of the webbing to restrain and protect an occupant.
There has also been proposed a seat belt retractor including an energy absorbing mechanism which, if a load acting on the webbing exceeds a preset predetermined value, causes a predetermined amount of pay out of the webbing under a predetermined load by the use of the twisting deformation of a torsion bar, thereby to absorb an impact energy occurring on the occupant.
A seat belt retractor of this type is disclosed in, for example, Japanese Patent Application Laid-Open No. 10-310027 (1998), which comprises a substantially tubular take-up drum on which webbing is wound, an emergency locking means for coupling a locking base to a retractor base to stop rotation in a direction in which the webbing is paid out in the event of a vehicle emergency, and an energy absorbing means for allowing the pay out of the webbing after the operation of the emergency locking means while applying a predetermined load to the webbing.
If a tension equal to or greater than the predetermined load acts on the webbing in the event of a vehicle emergency, the pay out of the webbing causes the take-up drum to move in the direction in which the webbing is paid out into contact with an inner peripheral portion of the retractor base. As the take-up drum is rotated by the pay out of the webbing resulting from the subsequent twisting deformation of the torsion bar, a grinding means provided on the inner peripheral portion of the retractor base cuts an outer peripheral surface of the take-up drum to absorb the energy. Additionally, the twisting deformation of the torsion bar also absorbs the energy.
However, the seat belt retractor disclosed in the above-mentioned background art document is structured so that the cutting by the grinding means is done after a free end portion of the take-up drum supported at one end in cantilevered fashion by the retractor base is moved to the aforementioned inner peripheral portion in the event of a vehicle emergency. Such a structure causes the start of the energy absorption by the grinding to be delayed by a time interval required for the movement until the take-up drum comes into contact with the inner peripheral portion, thereby decreasing the energy absorption immediately after the start of the pay out of the webbing. This increases the amount of forward movement of an occupant to create a danger that a secondary crash or collision against a steering or an instrument panel occurs and a danger that the impact force of the occupant upon an inflated air bag increases.
Further, because of the cantilevered structure, the take-up drum has a support structure such that the center of rotation thereof on the free end is movable. Hence, the take-up drum might be removed from the aforementioned inner peripheral portion by the force of contact with the inner peripheral portion of the retractor base or be engaged deeply into the grinding means. Moreover, under normal conditions, there is a danger that the take-up drum inadvertently moves to roughen a surface to be ground. In such cases, a problem arises that the grinding by the grinding means is not performed uniformly whereby stable energy absorption is not achieved.